function WithoutCurlyBraces() {
    if (true)
        for (let k in kv) {
            function foo() { return this }
            function bar() { return foo.call(this) }
            console.log(this, k) // => undefined
        }
}

function WithCurlyBraces() {
    if (true) {
        for (let k in kv) {
            function foo() { return this }
            function bar() { return foo.call(this) }
            console.log(this, k) // => 777
        }
    }
}